Table 1. Properties of substrates tested for growing pawpaw seedlings in containers in the greenhouse.

zMean separation within columns and fertilizer level by LSD, P < 0.05; n = 3. Mean ± SE

tration was 112 ± 1, 71 ± 10, 16 ± 2, 21 ± 1 mg·L-1, for ProMix, 1 sand : 1 sphagnum peat, 4 pine bark : 1 sand : 1 sphagnum peat, and 6 pine bark : 1 sand (v/v) substrates, receptively. The germination percentage was ≈ 80%, and no significant differences in this rate were detected among the four media (data not shown).

Analysis of variance indicated that there was a significant interaction between the main effects of potting substrate and fertilizer regime (P < 0.001) for shoot and total plant dry weight, but not for root dry weight (Table 2). The 100 mg·L-1 N fertilizer level significantly increased pawpaw seedling leaf number, height, shoot dry weight, total plant dry weight, and decreased root : shoot ratio in ProMix and 1 sand : 1 sphagnum peat (Table 2). In the 100 mg·L-1 N fertilizer treatment, seedling total dry weight production was significantly greater in ProMix than in 1 sand : 1 sphagnum peat (independent t test, P < 0.001; Table 2). Seedlings in 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat stopped growing and set terminal buds ≈ 50 d after fertilization treatments were imposed; however, seedlings did not display chlorosis in any substrate or fertilizer treatment. Root dry weight was significantly affected by medium type and fertilizer rate, and there was no interaction between the main effects. Root dry weight was similar in ProMix (2.3 g) and 1 sand : 1 sphagnum peat (2.1 g), and significantly (LSD, P < 0.05) greater in these media than in 6 pine bark : 1 sand (v/v) (1.3 g) and 4 pine bark : 1 sand : 1 sphagnum peat (1.6 g). Root dry weight was similar at 50 mg·L-1 N (1.9 g) and 100 mg·L-1 N (2.1 g) fertilization rates, but significantly (LSD, P < 0.05) less in non-fertigated seedlings (1.6 g).

(1.9 g) and 100 mg·L-1 N (2.1 g) fertigation rates, but significantly (LSD, P < 0.05) less in non-fertigated seedlings (1.6 g).

Expt. 2, root-zone heating study. Increasing the root-zone temperature from 24 to 32 °C by bottom heating significantly (independent t test, P < 0.001) hastened seedling emergence from the soil by 9 d. Seedlings reached the 2 to 3 leaf stage at 69 d after sowing with bottom-heat, but did not reach this stage in ambient plants until 79 d after sowing (Fig. 1A). Seedling leaf number increased steadily in both ambient and bottom-heated plants until harvest (Fig. 1A). Average seedling height of ≈10 cm was not achieved by ambient plants until 79 d after sowing, whereas seedlings with bottom heat reached this height 69 d after sowing (Fig. 1B).

Bottom heat increased all growth characteristics that were measured after harvest, except root: shoot ratio (Table 3). Bottom-heated seedlings had a significant increase in the following parameters relative to ambient plants: leaf number (30%), plant height (32%), whole plant leaf area (94%), shoot dry weight (104%), total root dry weight (50%), lateral root dry weight (125%), and total plant dry weight (87%; Table 3). Bottom-heated seedlings showed a decrease in root: shoot ratio of 25% and in SLW of 16% compared to ambient plants. Seedlings subjected to bottom heat had an increase in leaf chl concentration for chl a (39%), chl b (33%), chl p (43%), total chl (38%), and chl a : b ratio (8%) when compared to ambient seedlings (Table 3).

Discussion

• v)Growth characteristics of pawpaw seedlings in 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat were poor compared to plants in ProMix or 1 sand : 1 sphagnum peat. Mandujano and Fulbright (1998) reported that pawpaw seedlings in containers grew well and appeared very healthy in the pH range of 5.0 to 5.6, but at pH 5.7–6.0 they observed some leaf curling and tip burn. The pH range of substrate in this study was between 5.07 and 5.80 at the start of the experiment. Although the substrate pH may have changed during the experiment, differences in seedling growth response to the substrate used were not likely due to the differences in substrate pH. 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat tended to dry out quickly. Even though plants were watered daily, the lower water holding capacity of the pine bark may have reduced growth of plants in 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat. Wilting of plants was not observed in 6 pine bark : 1 sand (v/v) and 4 pine bark : 1 sand : 1 sphagnum peat, but plants in these substrates stopped growing and set terminal buds about halfway through the experiment, indicating that the plants experienced stress. Plants in the pine bark substrate may also have been stressed due to reduced nutrient availability; however, all plants appeared healthy at the end of the experiment. Cation exchange capacity for substrate mixtures used in container production of plants should be reported on a volume basis, due to the light weight of some media, and

Table 2. Growth characteristics of pawpaw seedlings grown in a greenhouse with four growing substrates 70 d after fertilizer application and 140 d after sowing.

2Mean separation within substrate and fertilizer level by LSD, P < 0.05.

NS, *, **, *** Nonsignificant or significant at P < 0.01, 0.001, and 0.0001, respectively.

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